1. Field of the Invention
The present invention relates generally to antilock brake systems. More particularly, the present invention relates to an electrically controlled hydrostatic motor and pump system for an antilock brake system.
2. State of the Art
An antilock brake system (ABS) is a control system for regulating vehicle braking to prevent locking of wheels during braking. During ABS control, the stability or instability of individual wheel motion is monitored. For example, wheel rotation is kept in a slip range which provides maximum braking force by controlling a succession of pressure build-up, pressure reduction and pressure holding phases.
Typically, an ABS includes wheel-speed sensors that monitor the motion of each vehicle wheel. If one wheel shows signs of locking, a sharp rise in peripheral wheel deceleration and in wheel slip occurs. If these values exceed predetermined critical values, an ABS controller commands a solenoid-valve unit in the ABS to stop or reduce wheel-brake pressure build-up until the danger of lock-up has passed. Afterwards, the brake pressure is again built-up to ensure that the wheel is not underbraked. A typical ABS includes solenoid valves which are repeatedly turned on/off to effect the aforementioned pressure build-up, pressure reduction and pressure holding phases. This solenoid actuated on/off control technique results in significant vibration and noise problems.
For example, a known ABS system available from Bosch is described in the Bosch Automotive Handbook, 2nd edition, 1986. Pages 528 to 532 describe an ABS system which includes a 3-channel hydraulic modulator for front-rear split brake circuits. Each channel of the Bosch ABS includes a three position solenoid valve and a return pump which is driven by an electric drive motor. In a first de-energized position of the solenoid valve, there is an unhindered passage of hydraulic fluid from a master cylinder to a wheel-brake cylinder when a brake pedal is activated (e.g., depressed). In this solenoid valve position, the wheel-brake pressure rises during initial braking and during automatic brake control. In a second, semi-energized position of the solenoid valve, hydraulic fluid passage from the master cylinder to a wheel-brake cylinder is interrupted. In this second position, the wheel-brake pressure is kept constant. In a third fully energized position of the solenoid valve, the wheel-brake cylinder is connected to the return pump and a return hydraulic line to decrease wheel-brake pressure.
Thus, the aforementioned conventional ABS uses repetitive solenoid actuation to increase, hold or decrease pressure in a wheel braking cylinder. The actuation signal for the solenoid is an on/off signal having a square wave configuration (i.e., vertical rise and fall times during activation/deactivation of the solenoid).
Similarly, the actuation signal for the electric drive of the return pump is conventionally a square wave which is used to either activate or deactivate the pump when a pressure decrease is commanded. The return pump operates in a single direction to redirect brake fluid via dampers to the master cylinder when brake fluid is to be removed from a wheel-brake cylinder (i.e., to reduce pressure). Pressure reduction typically lasts about 20 ms while pressure buildup lasts about 200 ms.
The conventional ABS described above suffers significant drawbacks. As mentioned above, because each of these systems includes solenoid controlled on/off brake pressure actuation in conjunction with on/off pump control, substantial vibration and noise occurs during ABS brake actuation. Further, the use of square wave signals to repeatedly activate/deactivate the solenoids and the one-directional pumps during ABS control results in prolonged settling time (i.e., hysterisis) before a wheel being controlled attains a desired command speed during a braking maneuver (i.e., a speed which provides maximum braking force with optimum slip).